1. Field of the Invention
The present invention relates to an active matrix substrate for use in a liquid crystal display device and the like, practically to an active matrix substrate which is used in combination with an opposing substrate having a counter electrode thereon and a display medium such as a liquid crystal interposed between the active matrix substrate and the opposing substrate.
2. Description of the Prior Art
Conventionally in a liquid crystal display device (hereinafter, referred to as the "LCD device"), an EL (electroluminescent) display device, a plasma display device and the like, a display pattern is formed on a display plane by selectively driving display pixels arranged in a matrix. As a system for selectively driving the display pixels, an active matrix driving system is known, according to which each independent pixel electrode is connected to a switching device. The active matrix driving system, which allows for high-contrast display, is used in a liquid crystal TV, a word processor, a computer and the like.
As the switching device for selectively driving the pixel electrodes, a TFT (thin film transistor), a MIM (metal-insulator-metal) device, a MOS (metal-oxide semiconductor) transistor, a diode, a varistor and the like is generally employed. Such a switching device is used to optically modulate a visible display medium such as a liquid crystal, an EL emitter, a plasma emitter and the like, thereby forming a visible display pattern on the display plane.
FIG. 11 is a plan view of a conventional active matrix substrate using an amorphous silicon (hereinafter, referred to as "a-Si") TFT having a reverse stagger structure as a switching device. In FIG. 11, a transparent insulating substrate 101 formed of glass or the like has a plurality of gate bus lines 121 arranged substantially in parallel and a plurality of source bus lines 122 arranged substantially in parallel. The gate bus lines 121 and the source bus lines 122 run perpendicularly to each other. A rectangular area defined by the two adjacent gate bus lines 121 and the two adjacent source bus lines 122 has a pixel electrode 140 formed therein. Each pixel electrode 140 is connected to a TFT 131a, and the TFT 131a has a drain electrode 133a on a rear surface of an end portion of the pixel electrode 140. The TFT 131a also has a source electrode and a gate electrode. The source electrode is constituted by a source bus branch line 122a branched from the source bus line 122, and the gate electrode is constituted by a portion 121a of the gate bus line 121. In other words, the TFT 131a is connected to one of the two gate bus lines 121 and to one of the two source bus lines 122.
FIG. 12 shows another conventional active matrix substrate using a TFT as a switching device. The gate bus line 121 has a gate bus branch line 121b branched therefrom, and a tip portion of the gate bus branch line 121b constitutes a gate electrode of the TFT 131a. The source bus line 122 has a projected portion 122a, which constitutes a source electrode of the TFT 131a. A drain electrode 133a of the TFT 131a is provided on a rear surface of an end portion of the pixel electrode 140.
FIG. 13 shows still another conventional active matrix substrate, in which each pixel electrode 140 is connected to two TFTs 131b and 131c each having the identical structure with that of the TFT 131a of FIG. 11. A drain electrode 133b of the TFT 131b and a drain electrode 133c of the TFT 131c are both provided on a rear surface of end portions of the pixel electrode 140.
FIG. 14 shows still another conventional active matrix substrate, in which each pixel electrode is connected with two TFTs 131b and 131c each having the identical structure as that of the TFT shown in FIG. 12. A drain electrode 133b of the TFT 131b and a drain electrode 133c of the TFT 131c are both provided on a rear surface of end portions of the pixel electrode 140.
FIG. 3b is an enlarged plan view of a connecting portion of the active matrix substrates of FIGS. 11 and 12, and FIG. 7b is an enlarged plan view of a connecting portion of the active matrix substrates of FIGS. 13 and 14. In FIG. 3b, the end portion of the pixel electrode 140 overlaps the drain electrode 133a. In FIG. 7b, the end portions of the pixel electrode overlap the drain electrodes 133b and 133c. It is known that such active matrix substrates have the following problems.
(1) It is possible that a portion of a step portion of the pixel electrode (the step portion being shaded in FIGS. 3b and 7b), which is formed on an end portion of the drain electrode, is undesirably thinner than the other portion as is shown in FIG. 15b. In FIG. 15a, which illustrates the pixel electrode overlapping the drain electrode in a satisfactory state, the step portion has the identical thickness with that of the other portion. The undesirable overlapping state shown in FIG. 15b tends to occur in the case when an end surface of the drain electrode is steeply inclined or the end portion of the drain electrode has an abnormal shape (for example, an edge thereof has an upward projection or the end surface is rough). Such an undesirable overlapping state of the step portion results in an increased resistance thereof and possibly a conduction failure. PA1 (2) In the case when the pixel electrode 140 is formed by photolithography, etching liquid possibly corrodes the step portion of the pixel electrode 140 and the drain electrode 133a (or 133b and 133c). Such corrosion occurs since a photo resist film formed on a pixel electrode material to obtain the pixel electrode 140 in a desired pattern is usually adhered onto the pixel electrode material at corners d and e (or f, g, h and i) in an inferior state than in the other portion. In the case when the whole step portion is corroded, the drain electrode 133a (or 133b and 133c) and the pixel electrode 140 are electrically separated from each other, thereby causing a conduction failure.
When a conduction failure occurs, the pixel electrode 140 is not supplied with current from the TFT 131a (or 131b and 131c). Accordingly, the liquid crystal interposed between the pixel electrode 140 and the counter electrode is not applied with a sufficient voltage for display. Such a phenomenon, which is recognized as a point defect in a display device, lowers the yield of the production. In the case when such a phenomenon occurs in a wide area, a great number of point defects are generated on the display device, resulting in a fatal reduction of the yield.
In order to solve the above problem, it is known to connect two TFTs to each pixel electrode as is shown in FIGS. 13 and 14. In such an active matrix substrate, even if a conduction failure occurs between one of the TFTs and the pixel electrode 140 due to contamination of foreign objects or a defective patterning of the photo resist film, the other TFT normally functions. Accordingly, the possibility that a point defect occurs can be lower than in the active matrix substrates in FIGS. 11 and 12.
However, it rarely occurs that a conduction failure is generated in one of the TFTs due to the aforementioned undesirable overlapping state or corrosion caused in the process of photolithography while the other TFT normally functions, since two adjacent TFTs are usually produced in extremely similar conditions. Accordingly, even if the pixel electrode 140 is connected with two or more TFTs, the generation of the point defect cannot sufficiently be prevented.